Question: $ A = \left[\begin{array}{rrrr}-1 & 0 & -1 & -2 \\ 1 & 0 & 4 & 1 \\ 3 & 4 & -1 & 1\end{array}\right]$ What are the dimensions of matrix $ A$ ?
Explanation: The first dimension is the number of rows in the matrix. How many rows does $ A$ have? The second dimension is the number of columns in the matrix. How many columns does $ A$ have? So, $ A$ is a 3 $\times$ 4 matrix.